APIPA is set bu default in windows and if your DHCP server can not find your IP address then it gets one assigned and it should start with 169.

Click to expand...

Actually, if your IP address is not static, it is not necessarily APIPA; it is dynamic. If a DHCP server is available, it will get the address information from the DHCP server. If a DHCP server is not available, a Windows device will assign itself an APIPA address.

Dynamic addresses don't necessarily change every time. You could very well get the same address multiple times in a row with DHCP. You lease an address from a DHCP server for a period of time. If you leave your computer off for longer than the lease duration, you might (or might not) pick up a different address.

Actually, if your IP address is not static, it is not necessarily APIPA; it is dynamic. If a DHCP server is available, it will get the address information from the DHCP server. If a DHCP server is not available, a Windows device will assign itself an APIPA address.

Dynamic addresses don't necessarily change every time. You could very well get the same address multiple times in a row with DHCP. You lease an address from a DHCP server for a period of time. If you leave your computer off for longer than the lease duration, you might (or might not) pick up a different address.

Actually, if your IP address is not static, it is not necessarily APIPA; it is dynamic. If a DHCP server is available, it will get the address information from the DHCP server. If a DHCP server is not available, a Windows device will assign itself an APIPA address.

Dynamic addresses don't necessarily change every time. You could very well get the same address multiple times in a row with DHCP. You lease an address from a DHCP server for a period of time. If you leave your computer off for longer than the lease duration, you might (or might not) pick up a different address.

Click to expand...

for instance i looked for it from command console and lease duration is an hour!

think i shut my pc for two hours , my IP will change or not change
in this way , let's assume it changed and applying the omitting rules of forum example ,

There will be two DHCP servers in this instance.
1. The first on on your modem/router, which will have a lease time for 1hour for example.
2. A DHCP server that is manager by your ISP, which probably has a lease time of 2 weeks for example.

say your ip address is 174.164.15.131 and you shut down your machine for 3 hours, and log back on, and the ip address is still 174.164.15.131, this is becuase your lease has not expired for the DHCP server that is managed by your ISP.

Remember,
your ip address 174.164.15.131, is your WAN address which is managed by your ISP.
your ip address 192.168.1.10, is your local ip address which is managed by your modem/router

As mentioned before, if your WAN ip address (174.164.15.131) did change your ISP can find out all of your previous ip address that were allocated to yourself.
Each time you log into a forum, just like this one, it stores your ip address on their records, and if you log under 2 separete usernames here and do some "dodgy" business, then, both user names will be assigned to that 1 ip address, as the ip address has not changed (unless you use an Anonymous (sp) proxy server)

There will be two DHCP servers in this instance.
1. The first on on your modem/router, which will have a lease time for 1hour for example.
2. A DHCP server that is manager by your ISP, which probably has a lease time of 2 weeks for example.

say your ip address is 174.164.15.131 and you shut down your machine for 3 hours, and log back on, and the ip address is still 174.164.15.131, this is becuase your lease has not expired for the DHCP server that is managed by your ISP.

Remember,
your ip address 174.164.15.131, is your WAN address which is managed by your ISP.
your ip address 192.168.1.10, is your local ip address which is managed by your modem/router

As mentioned before, if your WAN ip address (174.164.15.131) did change your ISP can find out all of your previous ip address that were allocated to yourself.
Each time you log into a forum, just like this one, it stores your ip address on their records, and if you log under 2 separete usernames here and do some "dodgy" business, then, both user names will be assigned to that 1 ip address, as the ip address has not changed (unless you use an Anonymous (sp) proxy server)

Does this make sense?

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you said "your ip address 192.168.1.10, is your local ip address which is managed by your modem/router"

but there are two local addresses , one of them is ethernet interface of pc and other is
ethernet interface of modem

and here you said

"
There will be two DHCP servers in this instance.
1. The first on on your modem/router, which will have a lease time for 1hour for example.
2. A DHCP server that is manager by your ISP, which probably has a lease time of 2 weeks for example.

you said "your ip address 192.168.1.10, is your local ip address which is managed by your modem/router"

but there are two local addresses , one of them is ethernet interface of pc and other is
ethernet interface of modem

and here you said

"
There will be two DHCP servers in this instance.
1. The first on on your modem/router, which will have a lease time for 1hour for example.
2. A DHCP server that is manager by your ISP, which probably has a lease time of 2 weeks for example.

"

simultaneously two dhcp servers?

Click to expand...

Your ISP assigns a public address to the external interface of your modem... and your modem assigns an internal address to your PC. So, yes, there are essentially two DHCP servers.

The internal Ethernet interface of your modem will have a static address. The address assigned to your PC will be on the same subnet as the internal interface of your modem so they can talk to each other.

For the record... all this stuff is covered in that Network+ book we advised that you study...

for instance i looked for it from command console and lease duration is an hour!

think i shut my pc for two hours , my IP will change or not change
in this way , let's assume it changed and applying the omitting rules of forum example ,

can they detect me? cause my IP changed!

Click to expand...

As Harry said, yes, your ISP still knows who you are based on the hardware address of your modem.

If the lease given to your modem is a one hour lease, and you shut the modem for two hours, then it might pick up a new address... or you might get the original address... it often depends on whether another device on that network is assigned your original address or not.

The DHCP lease process is also covered in detail on the Network+ exam... thus, why we recommended it to you.

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